Antigenic similarities among human and six nonhuman primate chorionic gonadotropins have been tested in homolgous radioimmunoassays and bioassay utilizing hCG subunits (alpha and beta) and antisera to these subunits as reagents. Marked antigenic similarities were found among those subunits and the chorionic gonadotropins of the chimpanzee, gorilla and orangutan. No such antigenic similarities were recorded for the baboon, macaque or marmoset. In the development of models for testing the advisability of specific vaccination regimens in man using fragments of hCG to achieve fertility control, the chimpanzee appears to be the optimal nonhuman primate.